1. Field of the Invention
The present invention relates to a method for forming a lens at a position corresponding to a photoelectric conversion section for converting light to an electric signal and/or converting an electric signal to light. The present invention further relates to a method for manufacturing a semiconductor apparatus such as a solid-state image capturing element, which uses the lens formed using the method for forming a lens and which is constituted of a plurality of light receiving sections for performing a photoelectric conversion on and capturing an image of incident light, as well as a light emitting section and a light receiving section. The present invention further relates to an electronic information device, such as a digital camera (e.g., a digital video camera or a digital still camera), an image input camera (e.g., a monitoring camera and a car-mounted camera), a scanner, a facsimile machine, a television telephone device, a camera-equipped cell phone device; which electronic information device includes the solid-state image capturing element, being the semiconductor apparatus manufactured using the method for manufacturing a semiconductor apparatus, as an image input device used in an image capturing section thereof; or relates to an electronic information device such as a pick-up apparatus, including the light emitting section and light receiving section as the semiconductor apparatus in an information recording and reproducing section thereof.
2. Description of the Related Art
Solid-state image capturing apparatuses, as conventional semiconductor apparatuses of this type, include CCD (Charge Coupled Device) image sensors and CMOS (Complementary Metal Oxide Semiconductor) image sensors and the like. These apparatuses are applied to various kinds of devices including a digital camera, a video camera, a scanner, a digital copying machine, a facsimile machine, or a camera-equipped cell phone device.
Further, with the spread of products of the conventional solid-state image capturing apparatus, demands have been increasing for the reduction of size and lowering of price in addition to higher performance such as increasing the number of pixels and improving light receiving sensitivity.
When advancements are made in reducing the size and increasing the number of pixels in the conventional solid-state image capturing apparatus and at the same time, the lowering of the price is demanded, the pixel size is reduced even more. Along such a reduction of the pixel size, one aspect of the fundamental performance of the solid-state image capturing apparatus: the light receiving sensitivity is lowered. This makes it difficult to capture a clear image in low luminous intensity. As a result, it is an important issue of how to improve the light receiving sensitivity per unit pixel.
Accordingly, for a method of improving the light receiving sensitivity of the conventional solid-state image capturing apparatus, a microlens is provided as a light focusing convex lens for each light receiving section in order to increase the light focusing efficiency of the light receiving section. There exist various kinds of methods for manufacturing such a microlens. As a first method in the conventional technique, Reference 1 discloses a method for forming a microlens in a desired shape with suitable reproducibility and at a low cost.
The method for forming a microlens disclosed in Reference 1 will be described with reference to FIGS. 6 and 7.
FIG. 6 is an essential part longitudinal cross sectional view illustrating a microlens forming step of a conventional CCD disclosed in Reference 1. FIG. 7 is an essential part longitudinal cross-sectional view of the CCD, illustrating a null region (distance X) between microlenses that are formed through the microlens forming step in FIG. 6.
As illustrated in FIGS. 6 and 7, each pixel section of a conventional CCD 100 includes: a semiconductor substrate 101; a photodiode section 102 provided on the semiconductor substrate 101, for performing a photoelectric conversion on incident light to generate a signal charge, as a light receiving element; and an electric charge transferring section 104 adjacent to each photodiode section 102, for reading out and transferring the signal charge in a predetermined direction from the photodiode section 102 through a read-out gate section 103.
In addition, above the read-out gate section 103 and electric charge transferring section 104, a gate electrode 105 is disposed with a gate insulation layer 106 interposed therebetween. The gate electrode 105 functions as an electric charge transferring electrode for controlling the transferring of electric charges.
Above the gate electrode 105, a light shielding layer 107 is formed with a gate insulation layer 108 interposed therebetween, for preventing noise (smear) from generating due to the reflection of incident light from the gate electrode 105. Above the photodiode section 102 (light receiving section), an inner-layer lens 110 is disposed for focusing light from an interlayer insulation layer 109 through an opening 107a of the light shielding layer 107 to the photodiode section 102. Further, a planarization layer 111 is formed on the inner-layer lens 110.
A thermoplastic resin for the microlens is applied on the planarization layer 111, wherein the resin has photosensitive and thermosetting characteristics. As such, the thermoplastic resin layer for the microlens, applied on the planarization layer 111, is exposed to be developed using a photomask of a predetermined pattern. As illustrated in FIG. 6, a thermoplastic resin layer 112 of a rectangular pattern form is thus formed at a position corresponding to each light receiving section. Thereafter, an appropriate amount of ultraviolet rays are irradiated onto the thermoplastic resin layer 112 in order to increase the transparency thereof.
Then, the thermoplastic resin layer 112 is heated to the temperature of the softening point or above to be thermally flowed. The thermoplastic resin layer 112 is further heated to at or above the temperature of the hardening point to form a microlens 112a with a desired convex lens shape as illustrated in FIG. 7, owing to the balance among the surface tension of the thermoplastic resin layer 112, the gravity acting thereon, and the curing characteristics of the thermoplastic resin layer 112. In addition, note that 113 denotes a stopper layer for separating elements.
Next, a method for forming a microlens by transferring disclosed in Reference 2 will be described in detail with reference to FIG. 8.
FIG. 8 is a longitudinal cross sectional view illustrating an exemplary structure of an essential part of a conventional CCD disclosed in Reference 2. Note that the members having the same function and effect as the corresponding constituent members in FIG. 7 are added with the same reference numerals, but their explanations will be omitted.
As illustrated in FIG. 8, a CCD 120 of Reference 2 is provided with a microlens resin layer 121 on a planarized layer 111 and the resin layer is planarized. A photosensitive resist layer material is applied on the microlens resin layer 121. Further, using a photomask formed with a light shielding layer pattern with a gradually changed dot density, an applied photosensitive resist layer is exposed and developed to form a photosensitive resist layer 122 which is patterned in a microlens shape. Then, an entire wafer surface including the photosensitive resist layer 122 is subjected to an atmosphere in which an organic layer is etched (etch back method), to transfer the shape of the photosensitive resist layer 122, which is patterned as the above-described microlens shape, to the microlens resin layer 121 below. At the same time with the transferring, the photosensitive resist layer 122 is removed.
Next, a method for forming a microlens disclosed in Reference 3 will be described in detail with reference to FIG. 9.
FIG. 9 is a longitudinal cross sectional view illustrating an exemplary structure of an essential part of a conventional CCD disclosed in Reference 3. Note that the members having the same function and effect as the corresponding constituent members in FIG. 7 are added with the same reference numerals, but their explanations will be omitted.
Reference 3 discloses a photomask and a method for preparing pattern data with the photomask. As illustrated in FIG. 9, the following is performed to form a microlens 132 on a planarized layer 111 in a CCD 130 of Reference 3: a resist in which remaining film thickness varies in accordance with the amount of exposure is used for development; an exposure amount distribution of a photomask pattern is determined to obtain a desired profile; a photomask pattern forming plane is defined with X-Y coordinates; coordinate values x and y are defined as a function; an aimed transmitted light amount (exposure amount) distribution of a photomask is expressed as a value z on a z-coordinate which is orthogonal to the pattern forming plane (coordinate values x and y); the positioning of a dot pattern is determined by this process of realizing the transmitted light amount (exposure amount) distribution; and a process of generating and positioning the dot pattern is performed on a region in the X-Y coordinates, where the pattern is determined to be positioned.
Accordingly, by using the method of preparing pattern data to form a photomask for the manufacturing of a minute light-focusing lens array (microlens 132) on the upper side of a light receiving section of an image sensor, such as CCD and CMOS image sensors, it becomes possible to manufacture a photomask which is capable of forming an aimed resist shape after development with suitable reproducibility and accuracy.    Reference 1: Japanese Patent No. 2945440    Reference 2: Japanese Patent No. 3158296    Reference 3: Japanese Laid-Open Publication No. 2004-70087